This invention relates to a method and apparatus for projection welding metal bodies together.
FIG. 1 shows a schematic side view of a conventional projection welding apparatus useful, for example, to weld inner and outer annealed steel door plates 1a and 1b together in order to reinforce the outer plate. In FIG. 1, a hemispherical projection 2 is formed on the inner plate 1a; the outer plate 1b is flat and planar, at least in the vicinity of the weld connection to be made. Reference numeral 3a designates an upper electrode for applying a welding current pulse to the inner plate 1a, 3b is a lower electrode fixed to a base stand 4 for applying a welding current pulse to the outer plate 1b, and 5 is an air cylinder fixed to an upper stand 6 for pressing the upper electrode 3a downwardly through a rod 7. The upper and lower electrodes are connected to a power source 8, and are insulated from the stands 4, 6. The power source 8 is typically either a single phase alternating current power source or a condenser discharge power source.
FIG. 2 shows the current waveform from a suitable single phase alternating current power source, with the ordinate designating the current in kilo-amperes and the abscissa designating the time in milli-seconds. FIG. 3 shows the current waveform from a suitable condenser discharge power source wherein the time (ta) for the current to reach a maximum value is 10 milli-seconds. As before, the ordinate designates the current in kilo-amperes and the abscissa designates time in milli-seconds.
In the use of the conventional projection apparatus shown in FIG. 1, the projection 2 on the inner metal plate 1a is abutted against the outer metal plate 1b, whereafter both plates are urged together by pressurizing the air cylinder 5 to a predetermined level and the power source is simultaneously triggered to apply a high energy welding signal across the electrodes 3a, 3b. A current thus flows from the plate 1a to the plate 1b through the projection 2 and is concentrated at the contact interface between the projection and the outer plate. The Joule heat generated thereby quickly fuses the inner and outer plates together.
In such a conventional projection welding process the relatively long power application time, from 20.about.400 milliseconds, causes the periphery of the welding zone to become overheated, which thermally distorts the outer plate 1b.
FIG. 4 shows a sectional view of a weld made according to the conventional process, wherein reference numeral 9 designates a welding nugget formed by the fused projection 2, 10 is the rear portion of the inner plate, and 11 is a surface distortion of the outer plate 1b caused by the thermal contraction of the welding nugget. The depth of such distortion is typically 20.about.40 .mu.m which detracts from the surface appearance of the plate 1b, and this necessitates the application of a filler composition and the subsequent shaving thereof pursuant to surface finishing or painting.